Growers provide guidance how large a species can get — in the case of Thuja 'Green Giant', probably about 18' wide — under ideal conditions. But a local nursery might give better advice on how large it will get, in your area and under your conditions. So I'd start there; and if you're uncertain, cheat toward the ideal a little bit. In this case, if you expect a "true" width of 12', then planting the trees 6' to 8' from the edge of your driveway or fence should prevent significant overhang or damage. Similarly, planting them 6' to 8' on center would create an edge-to-edge hedge at maturity.